feat: add multi-model delegation (Phase 0) and context compaction (Phase 1)

Phase 0 — Multi-Model Delegation:
- AgentOrchestrator wraps NativeAgent with delegate() for stateless
  single-turn calls to any model tier (fast/default/complex/local)
- DelegationConfig maps task types (compaction, classification, etc.)
  to model tiers
- Delegation prompts for compaction, memory extraction, classification,
  and tool summarisation
- Per-tier usage tracking for cost visibility
- Config schema: agents.delegation and agents.primary_tier

Phase 1 — Context Compaction:
- Token estimation (char/4 heuristic) with context window lookup
- shouldCompact() threshold check against context window percentage
- compactHistory() splits old/recent messages, delegates summary to
  fast tier, returns CompactionResult
- Automatic compaction in AgentOrchestrator.process() when configured
- Force-compact via orchestrator.compact() with session persistence
- Session.replaceHistory() with atomic SQLite transaction
- /compact TUI command with feedback on compacted token counts
- Config schema: compaction.enabled, threshold_pct, keep_turns,
  summary_max_tokens

Tests: 385 passing across 50 files (22 new tests in 2 new test files)

src/frontends/tui/commands.test.ts

@@ -26,6 +26,10 @@ describe('parseCommand', () => {
     expect(parseCommand('/fs')).toEqual({ type: 'fullscreen' });
   });
 
+  it('parses /compact command', () => {
+    expect(parseCommand('/compact')).toEqual({ type: 'compact' });
+  });
+
   it('parses /model command without argument', () => {
     expect(parseCommand('/model')).toEqual({ type: 'model' });
   });
@@ -64,6 +68,7 @@ describe('getHelpText', () => {
     expect(help).toContain('/help');
     expect(help).toContain('/model');
     expect(help).toContain('/reset');
+    expect(help).toContain('/compact');
     expect(help).toContain('/quit');
   });
 });

src/frontends/tui/commands.ts

@@ -4,6 +4,7 @@ export type Command =
   | { type: 'help' }
   | { type: 'status' }
   | { type: 'fullscreen' }
+  | { type: 'compact' }
   | { type: 'model'; name?: string }
   | { type: 'backend'; provider?: string }
   | { type: 'transfer'; target: string }
@@ -38,6 +39,11 @@ export function parseCommand(input: string): Command | null {
     return { type: 'fullscreen' };
   }
 
+  // Compact
+  if (trimmed === '/compact') {
+    return { type: 'compact' };
+  }
+
   // Model (with optional argument)
   if (trimmed === '/model') {
     return { type: 'model' };
@@ -73,6 +79,7 @@ Commands:
   /model [name]          Show or switch model (local, default, fast, complex)
   /backend [provider]    Show or switch local backend (ollama, llamacpp)
   /reset, /clear, /new   Clear conversation history
+  /compact               Compact conversation history
   /status                Show session info and token usage
   /fullscreen, /fs       Switch to fullscreen mode
   /transfer <dest>       Transfer session to another frontend
@@ -90,6 +97,7 @@ export const SLASH_COMMANDS = [
   '/reset',
   '/clear',
   '/new',
+  '/compact',
   '/status',
   '/fullscreen',
   '/fs',
@@ -106,6 +114,7 @@ export const COMMAND_TOOLTIPS: Record<string, string> = {
   '/reset': 'Clear conversation history',
   '/clear': 'Clear conversation history',
   '/new': 'Start a new conversation',
+  '/compact': 'Compact conversation history to save context space',
   '/status': 'Show session info and token usage',
   '/fullscreen': 'Switch to fullscreen mode',
   '/fs': 'Switch to fullscreen mode',

src/frontends/tui/minimal.test.ts

@@ -44,6 +44,7 @@ describe('MinimalTui backend command', () => {
       getHistory: () => [],
       addMessage: vi.fn(),
       clear: vi.fn(),
+      replaceHistory: vi.fn(),
     };
 
     const mockRouter = {
@@ -84,6 +85,7 @@ describe('MinimalTui backend command', () => {
       getHistory: () => [],
       addMessage: vi.fn(),
       clear: vi.fn(),
+      replaceHistory: vi.fn(),
     };
 
     const mockRouter = {